Page 11 - DISS_NYRKOVA

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internal factors. Shot blasting treatment of the surface reduces the difference in

the susceptibility of pipe steel of a different strength to corrosion cracking: at a

protective potential of -1.0 V, which is close to the maximum, this difference,

evaluated by the coefficients K is reduced from 2.2 for ground surfaces to
τ

1.1 for the surfaces after treatment.

It was for the first time revealed that cathodic disbondment of

polyurethane and epoxy coatings from X70 and X80 steels at a potential of

o
-1.45 V in 3% NaCl at a temperature of 20 C is affected only by the properties
of the coatings and solution. However, at a cyclic change in temperature (from

о
о
20 С to 50 С), cathodic disbondment of coatings from X80 steel runs more
intensely than from X70, which is confirmed by larger disbondment radii: 16.6

mm for Х70, 23.1 mm for Х80 for polyurethane coating, 17.5 mm for Х70,

23.4 mm for Х80 for tape coating, 8.5 mm for Х70 and 9.3 mm for Х80 for

epoxy coating. The observed regularity is predetermined by the peculiarities in

the structure of a near-surface steel layer, in particular, by the reduction in the

hydrogen evolution potential to -0.850 V as compared to X70 steel, for which

this potential is equal to -0.950 V.

According to the results of a complex of corrosion-mechanical,

fractographic and electrochemical studies, it was proved that there are three

potential areas, in which the corrosion cracking of X70 steel in the model soil

electrolyte occurs by different mechanisms. In the region of potentials, which

are more positive than -0.75 V, corrosion cracking occurs by the mechanism of

local anodic dissolution. At the potentials from -0.75 V to -1.05 V, local anodic

dissolution and hydrogen embrittlement occur simultaneously. At the potentials

lower than -1.05 V, the predominant mechanism of corrosion cracking is

hydrogen embrittlement. The change in the mechanism of corrosion cracking

depending on the potential is confirmed by the change in mechanical properties,

fractography of fracture surfaces and susceptibility to corrosion cracking,

evaluated by the coefficient K .
S